1. Field of the Invention
The present invention relates to a honeycomb filter and a method for producing a honeycomb filter.
2. Discussion of the Background
The number of automobiles has increased drastically this century. As a result, the amount of gas discharged from automobile engines has continued to increase proportionally. Various substances suspended in the exhaust gas that is emitted, especially from diesel engines, cause pollution and severely affect the environment. Further, recently reported research results have shown that the fine particles suspended in gas emissions (diesel particulates) may cause allergies or decrease sperm counts. Thus, actions to eliminate the fine particles suspended in gas emissions must immediately be taken for the sake of mankind.
Due to this situation, many exhaust gas purification apparatuses have been proposed in the prior art. A typical exhaust gas purification apparatus includes a casing, which is located in an exhaust pipe connected to an exhaust manifold of an engine, and a filter, which is arranged in the casing and has fine pores. In addition to a metal or an alloy, the filter may be formed from ceramic. A cordierite honeycomb filter is a known example of a ceramic filter. Recent filters are often formed from sintered porous silicon carbide body that is advantageous from the viewpoints of heat resistance and mechanical strength, has a high accumulating efficiency, is chemically stable, and has a small pressure loss.
The pressure loss refers to the difference between the pressure value taken upstream of the filter and the pressure value taken downstream of the filter. A main cause of power loss is the resistance the exhaust gas encounters when passing through a filter.
The honeycomb filter includes a plurality of cells extending along the axial direction of the honeycomb filter. When the exhaust gas passes through the filter, the walls of the cells trap fine particles. This removes fine particles from the exhaust gas.
However, the honeycomb filter, which is made of a sintered porous silicon carbide body, is vulnerable to thermal impacts. Thus, larger filters are liable to crack. Accordingly, a technique for manufacturing a large ceramic filter assembly by integrating a plurality of small filters has recently been proposed to prevent breakage resulting from cracks.
A typical method for manufacturing a ceramic filter assembly will now be discussed. First, ceramic raw material is continuously extruded from a mold of an extruder to form an elongated square honeycomb molded product. After the honeycomb filter is cut into pieces of equal length, the cut pieces are sintered to form a filter. Subsequent to the sintering process, a plurality of the filters are bundled and integrated by adhering the outer surfaces of the filters to each other with a ceramic seal layer having a thickness of 4 to 5 mm. This completes the desired ceramic filter assembly.
A mat-like thermal insulative material, made of ceramic fiber or the like, is wrapped about the outer surface of the ceramic filter assembly. In this state, the assembly is arranged in a casing, which is located in an exhaust pipe.
However, in the prior art, there is a shortcoming in that the fine particles trapped in the ceramic filter assembly do not burn completely and some of the fine particles remain unburned. Accordingly, the efficiency for processing the exhaust gas is low.
Further, the honeycomb filter of the prior art has corners. Thus, there is a tendency of stress concentrating on the corners of the outer surface and chipping the corners. Further, the seal layer may crack and break the ceramic filter assembly from the corners thereby damaging the entire ceramic filter assembly. Even if the assembly does not break, there is a shortcoming in that leakage of the exhaust gas may decrease the processing efficiency.
During usage of the filter assembly, a high temperature difference between the honeycomb filters may cause thermal stress to crack the honeycomb filters and break the entire assembly. Thus, the strength of each honeycomb filter must be increased to increase the strength of the honeycomb filter assembly.
The prior art ceramic filter assembly as a whole has a rectangular cross-section. Thus, the periphery of the assembly is cut so that the assembly as a whole has a generally round or oval cross-section.
However, the filter has a plurality of cells. Thus, if the periphery of the assembly is cut, the cell walls are exposed from the peripheral surface subsequent to the cutting. This forms lands and pits on the peripheral surface. Thus, even if the assembly is accommodated in the casing with the thermal insulative material attached to the peripheral surface of the assembly, gaps are formed in the longitudinal direction of the filters. Thus, exhaust gas tends to leak through the gaps. This lowers the processing efficiency of the exhaust gas.
With regard to diesel particulates trapped in the honeycomb filter, it has been confirmed that particulates having a small diameter have a high lung attaching rate and increase the risk to health. Thus, there is great need to trap small particulates.
However, when the pore diameter and the porosity of the honeycomb filter are small, the honeycomb filter becomes too dense and hinders smooth passage of the exhaust gas, which, in turn, increases the pressure loss. This lowers the driving performance of the vehicle, lowers fuel efficiency, and deteriorates the driving performance.
On the other hand, if the pore diameter and porosity rate are increased, the above problems are solved. However, the number of openings in the honeycomb filter becomes too large. Thus, fine particles cannot be trapped. This decreases the trapping efficiency. Further, the mechanical strength of the honeycomb filter becomes low.